In oil and gas exploration it is important to obtain diagnostic evaluation logs of geological formations penetrated by a wellbore drilled for the purpose of extracting oil and gas products from a subterranean reservoir. Diagnostic evaluation well logs are generated by data obtained by diagnostic tools (referred to in the industry as logging tools) that are lowered into the wellbore and passed across geologic formations that may contain hydrocarbon substances. Examples of well logs and logging tools are known in the art. Examples of such diagnostic well logs include neutron logs, gamma ray logs, resistivity logs and acoustic logs.
Logging tools frequently are used for log data acquisition in a wellbore by logging in an upward (up hole) direction, from a bottom portion of the wellbore to an upper portion of the wellbore. The logging tools, therefore, need first be conveyed to the bottom portion of the wellbore and then pulled upwards through the wellbore. In many instances, wellbores can be highly deviated, or can include a substantially horizontal section.
During drilling, drilling mud fills the borehole. The pressure of the drilling mud is maintained at a pressure greater than that of the formation to keep the formation fluid within the formation. The drilling mud contains solid particles that build up on the wellbore and form a mudcake. The differential pressure used during drilling is generally maintained sufficient to stop an inflow of oil or gas into the wellbore during drilling operation which under certain conditions could result in an uncontrolled well (e.g., a “blow out”).
As the logging tool is lowered or raised within the formation, a flow of fluid occurs around the tool. This flow can dislodge the mudcake, and the tool can become lodged against one of the geologic formations because of differential pressure between the wellbore and the formation. Several factors increase the likelihood of sticking, including tool length, high permeability of the reservoir, deviated wellbores, and poorly formed mudcakes. In addition, the longer a tool stops within a wellbore, the greater the likelihood of the tool becoming lodged. Further, the wire or cable used to raise and lower the logging tool can become lodged in a wellbore. Although it does not have as great a cylindrical surface area as a logging tool, the wire has much more length. An added complication is that attempts to pull the stuck wire out of the formation can result in the wire beginning to cut into the formation (especially when the wellbore is deviated from vertical), which makes the wire—and the tool—lodged more tightly.
Current methods to address sticking of tools as a result of differential pressure are primarily preventative measures. These efforts include recirculating the mud to rebuild the mudcake and centralizing the tool. After a tool has become lodged, breakaways located on the tool itself are used. The use of the breakaways, however, only results in the retrieval of part of the tool, rather than the entire tool. The remaining portion of the tool can result in potential future problems.